Field of the Invention
The invention relates to an electrical machine with a rotor, mounted rotatably about a machine axis, and with a stator. The electrical machine is in particular an axial flux motor.
Description of the Background Art
Motor vehicles are being increasingly powered by electric motors, which are designed, for example, as wheel hub motors. To increase motor vehicle efficiency and dynamics, it is necessary that the electric motors are designed to be relatively compact and to have a relatively low weight. So-called axial flux motors are particularly suitable for this. These typically have a number of electrical windings, which are arranged parallel to the rotor's rotation axis and around it with the formation of a gap. A magnetic field parallel to the rotation axis is produced by the electrical windings, whereby the particular magnetic pole is spaced apart from the rotation axis and extends in the axial direction. It acts together with the permanent magnets, comprising the rotor, which is generally arranged in the axial direction in front of the stator comprising the electrical windings. In this regard, none of the electrical coils have a winding head, for which reason all winding elements are used to generate the magnetic field driving the rotor. In the case of an axial flux motor, a relatively fine pole pitch is realizable, so that the rotor can also be operated with a relatively low speed and a relatively large torque, which is a use profile of a wheel hub motor. No gears are necessary for this, which reduces friction losses, on the one hand, and weight, on the other. Manufacturing costs are also reduced in this case.
In comparison with radial flux motors, which have a number of electrical windings which are arranged perpendicular to the rotation axis and by means of which a magnetic field oriented in the radial direction is provided, the design of the stator is relatively complex, however. For this purpose, typically the winding-bearing part of the stator is produced by a sintering process, for which purpose ferrite powder is generally pressed into a suitable mold. In this way, forming of the desired magnetic field is relatively precise and the production is also relatively cost-effective. However, the electrical machine provided with a stator of this type has reduced magnetic properties particularly in the low-frequency range, for which reason efficiency is reduced. As a result, a larger stator is necessary to achieve a specific performance. Moreover, the mechanical integrity of the stator is relatively low.
An alternative is the winding of the stator comprising band strips made of electrical sheet, which is rolled up in a spiral form. Because the recesses, which form the later stator slots, must be present even before the winding process, and because the radius and consequently the tangential gap between adjacent recesses, forming the later slots, increase with an increasing radius, it is necessary that the band strip is fabricated adapted to the electric motor. In other words, the gap must always be determined anew between each punching. Because there always is play during the rolling to form the stator, the individual recesses are not completely aligned, for which reason a cost-intensive post-processing of the stator is necessary for realizing relatively low fabrication tolerances.